The present invention relates to an apparatus and method for removing ionic and particulate impurities in used hydrogen fluoride to provide purified hydrogen fluoride having impurities in the parts per billion range.
Hydrofluoric acid or hydrogen fluoride (HF) has many industrial uses related to its most notable property, the ability to dissolve silica, such as quartz and glass. Hydrogen fluoride is widely used in the semiconductor industry, for instance, in cleaning quartz diffusion tubes, in wafer cleaning, and in etching processes. Contaminants build up in the hydrogen fluoride as it is used, and when an unacceptable level of impurities is present, the HF is presently discarded.
Hydrogen fluoride is a relatively expensive material, and the raw material cost for replacing discarded or spent HF is substantial. Moreover, HF is considered to be a hazardous waste material. Accordingly, disposal of spent or contaminated HF solutions is itself a costly proposition because of the special procedures that must be followed in properly disposing of this material.
Accordingly, there is a need to reduce the HF raw material and disposal costs in manufacturing and other processes that utilize HF therein, but that require HF having a relatively high level of purity. The ability to use and reuse HF almost indefinitely is a particularly attractive proposition.
The semiconductor industry, perhaps more than any other industry, is committed to effective contamination control. The production of state of the art semiconductor devices requires strict contamination control in the environment, the equipment and the chemicals utilized therein. Semiconductor manufacturing equipment is constructed of non-contaminating materials, and often includes robotic wafer handling to minimize particulate contamination. Cleanrooms are essential to maintenance of particulate-free environments during the semiconductor fabrication process.
In addition to manufacturing equipment and environment, the third major potential source of particulate contamination in semiconductor fabrication processes is the process chemicals themselves.
Semiconductor fabrication typically involves contacting a silicon wafer with various liquids for an aggregate total of 20 or more hours. Of course, the purity of these liquids is of great concern, because ionic or particulate impurities in solutions contacting semiconductor wafers can drastically reduce yield and increase costs.
Thus, it is apparent that there is a need for an effective apparatus and method for repurifying HF to provide ultrapure HF having impurities in the parts per billion (ppb) range.
Vulikh, et al., U.S. Pat. No. 4,056,605, disclose a method for separating hydrofluoric acid from fluosilicic acid (H.sub.2 SiF.sub.6), by passing HF through an anion exchange column, to obtain HF containing fluosilicic acid and sulfuric acid in concentrations no greater than 0.5% to 1% by weight. The purified HF is said to be useful in production of synthetic cryolite and aluminum fluoride.
Removal of fluosilicic acid from a mixed acid stream through use of an anion exchange resin is further disclosed by Hiwatashi, U.S. Pat. No. 3,383,324. The purified acid solution is used for pickling silicon steel sheets. A similar disclosure is made by Hiwatashi, British Pat. No. 1,126,232.
Although the various prior art HF purification methods are effective in reducing the level of fluosilicic acid impurity and some other impurities to a sufficiently low level to permit reuse of the HF in many chemical processes, none of the prior art HF purification methods provide the ultrapure HF required by the semiconductor industry and other high technology industries.
Accordingly, it is an object of the present invention to provide a method and apparatus for purifying and regenerating HF to provide an end product having total impurities in the parts per billion (ppb) range.
Another object of the present invention is to provide a method for purifying and regenerating HF that will permit almost indefinite reuse of HF, allowance being made for minor processing losses and the need to make up for small amounts of HF actually consumed in use. Such a method would almost eliminate the substantial raw material cost of replacement HF in modern semiconductor fabrication processes.
It is a further object of the present invention to provide a method and apparatus for HF regeneration that is suitable for on-site use in semiconductor fabrication facilities for small or large scale HF purification.
A further object of the invention is to provide a method and apparatus that removes anionic impurities, cationic impurities, metallic impurities and particulate impurities from HF down to the ppb range.
Still a further object of the present invention is to provide an apparatus and method utilizing automatic controls to ensure the purity of the end product.
Other objects, features and advantages of the present invention will be apparent from the following Summary of the Invention and Detailed Description of the Invention.